The present invention relates to radar controlled weapons systems and, more particularly, to a method and apparatus for generating real-time high resolution Synthetic Aperture Radar (SAR) imagery from an airborne platform of a translating ship under the influence of roll, pitch, and yaw motions characteristic of sea state conditions.
A highly resolved ship image on an airborne display permits targeting to a particular part of the ship for the purpose of standoff command guidance weapon delivery. As such, this invention relates to airborne SAR systems used for generating real-time high resolution imagery of a ground target area and for accurately measuring and tracking the range and azimuth angle of a designated single non-emitting stationary or moving target cell within the aforesaid displayed target area so as to enable the accurate delivery of an air-to-ground missile or glide bomb from the SAR bearing aircraft to said target. A system for accomplishing the above has previously been described in a commonly assigned copending U.S. patent application of Sol Boles and Walter J. Smrek, Ser. No. 373,806, filed Apr. 30, 1982, entitled, "Moving Target Ordnance Control", the disclosure of which is incorporated herein by reference.
As disclosed in the aforementioned application of Boles and Smrek, a missile or glide bomb is brought to the start of a near-vertical terminal trajectory by inertial or radio location mid-course guidance techniques. In the terminal guidance phase, an interferometer antenna system is used in conjunction with a synthetic aperture radar and associated signal processing equipments to perform measurements of the relative range and azimuth angle between the weapon and a ground target designated by the operator on a high resolution real-time synthetic aperture ground map display. A command guidance channel from aircraft to weapon uses this sensor derived information to command the weapon trajectory in such a way as to drive the relative range and azimuth separation to zero so that the weapon impacts the target.
According to contemporary art, the generation of SAR imagery is predicated on the fact that when an aircraft flies over fixed terrain, a frequency gradient at constant range due to aircraft motion is produced. The processing of the integrated signal received from a small fixed range increment over a specified time interval through a contiguous doppler filter bank, after the application of suitable phase corrections of all such incoming signals required because of aircraft motion, enables the resolution of signal returns from amongst closely separated scatterers distributed in the azimuth dimension. Similar processing of signals received from closely separated contiguous range intervals permits the formation of an intensity plot on a CRT of the signals received from all scatterers in a range versus azimuth coordinate frame, consistent with the resolution limits of the system. Furthermore, the designation of any resolution cell within the map so formed, permits the measurement interferometrically of the azimuth angle of that cell to be used for command guidance of a weapon as described earlier. Resolution in the range direction depends straight-forwardly upon transmitted pulse width and the sampling rate of the received signal. Resolution in the cross-range, or azimuth direction, depends upon knowledge of aircraft cross line-of sight velocity, a measure of the rotation rate of the line-of-sight about the focus point so that doppler filter bandwidths and filter spacings can be preset in terms of desired azimuth angular resolution. Furthermore, the designation of any resolution cell within the map so formed by the placement of a cursor on the display derived from the mixing of injected artificial range and doppler signals with ground target video, permits the measurement interferometrically of the azimuth angle of the target cell as identified by the cursor, to be used for command guidance of a weapon as described earlier. Weapon impact accuracy requirements dictate that target tracking in range and azimuth be carried out over successive apertures during the weapon guidance to impact. This requires, in turn, that the cursor location on the range/doppler map be driven so that it remains on the designated target, so that the range and interferometric azimuth angle of the target are identified in a continuous manner. Such cursor tracking is accomplished on the basis of on-board sensor derived navigational measurements as the aircraft changes location with respect to the fixed ground terrain.
For the case of a ship undergoing roll, pitch, and yaw in sea state conditions, however, the net frequency gradient of the signal return within a given range bin becomes substantially determined by ship scatterer motion and could be markedly different from that due to aircraft motion alone. Moreover, as described in more detail below, the ship rotational motions accompanying sea state conditions can produce severe distortions in range/doppler ship imagery, thereby seriously undermining ship recognition capability and the successful execution of standoff command weapon guidance to a designated ship target cell using relative range and azimuth guidance techniques as delineated herein.
In view of the foregoing, it should be apparent that the known techniques which utilize the frequency gradient due to aircraft motion alone are unacceptable for generating real-time high resolution SAR imagery from an airborne platform of a translating ship under the influence of sea state conditions.
It is accordingly a general object of the present invention to overcome the aforementioned limitations and drawbacks associated with existing systems.
It is a specific object of the present invention to provide a method and apparatus for generating high resolution synthetic aperture radar displayed imagery of a ship under the influence of sea state conditions.
It is another object of the present invention to provide a method and apparatus for deterining the cross line-of-sight relative velocity associated with aircraft and ship rotational rates, requisite to achieving prescribed cross-range (azimuth) resolution in the displayed image representation of the ship.
It is a still further object of the present invention to provide method and apparatus for eliminating the distortions inherent in range/doppler ship imagery brought about by ship rotational motions accompanying sea state conditions by forming a scaled SAR high resolution range/cross-range (azimuth) image projection of the ship on the basis of interferometric azimuth angle measurements derived from doppler processed range sampled data, so as to permit the cursoring and tracking of a particular range/azimuth resolution cell of the ship target for the purpose of carrying out standoff command guidance weapon delivery to said target.